Vitamin A is necessary for maintenance of normal differentiation. As retinyl phosphate (Ret-P) this vitamin participates in mannosyl transfer reactions in a manner distinct from that of dolichylphosphate (Dol-P). Since amounts of Ret-P and Dol-P available for mannosylation can be assayed in the same incubation containing rat liver microsomes, the effect of different stages of differentiation and tumor growth rates on such ratio was determined, utilizing the Morris series as well as chemically induced hepatocellular carcinomas. The transplanted tumors had less than 5% the amount of A found in the host tissue. Moreover, a decrease in the Ret-P/Dol-P ratio from 0.3 in normal tissue to 0.1 in the moderately differentiated and 0.025 in the poorly differentiated and undifferentiated hepatomas was found. Such remarkable change correlated with the absence of the cellular retinol binding protein in these tumors. The Ret-P/Dol-P ratio in the tumor was found similar to that of microsomes from vitamin A-depleted liver tissue. The consequences of this deficiency in Ret-P on cell surface glycosylation are being investigated in vitamin A deficiency and in tumor tissue.